1. Field of the Invention
The present invention relates to a telescope or a binocular, and more specifically to a telescope having an automatic focusing function.
2. Description of the Prior Arts
Japanese published Patent Application No. S62-6205, Japanese published Patent Application No. S60-46407 and Japanese laid-open Patent Application No. S56-154705 disclose binoculars having the automatic focusing function.
In performing an automatic focusing in a telescope, it is considered to employ the TTL (through the lens) method which is frequently employed for cameras as a distance measuring method or a focus detecting method. In a binocular, however, since the aperture of a lens is generally small, if a light beam received through a principal optical system is used for a distance measurement or a focus detection, an object and the background become dark. Therefore, an external light method is preferable to the TTL method as the distance measuring method. Further, since a binocular is generally used for viewing a distant object, it is not desirable that a binocular emits a light beam for a distance measurement toward an object. Therefore, a passive method should be employed where simply a light beam originating from an object is received.
An article on "Olympus IZM300" is described on and after page 81 of a magazine "Camera Industry" (No. 6, 1988). The article, which is on a camera, shows a distance measuring method according to an external light passive method where a distance measuring lens is fixed. The driving source of the distance measuring lens employed therein is a direct current motor (hereinafter referred to as DC motor), and operations such as accelerating, racing and braking, etc. for controlling the DC motor with a high precision are performed while an actual lens driving position is being detected.
However, when the DC motor is employed as a driving source of the distance measuring lens, a feedback mechanism such as an encoder, etc. for detecting a lens position and returning it to the DC motor is required. Consequently, a circuit and software with resect to the feedback mechanism are required, whereby the cost increases and it becomes difficult to make a telescope compact. Further, since a telescope such as a binocular, etc. is frequently used for following and viewing an object, a continuous automatic focusing is preferable. In the DC motor, however, errors are easily accumulated when the lens is moved in both directions. Therefore, the DC motor is not appropriate for the continuous automatic focusing.
Further, in the binoculars according to the above-mentioned prior arts, the automatic focusing mechanism similarly works both to a distant object and to a close object. Therefore, for example, when a close object comes into the visual field of a binocular under a condition where the binocular is in focus on a distant object, the binocular tends to be focused on the close object.
However, when some obstacle (such as a person, a car, an animal, etc.) passes between the object and the user while a relatively distant object is being viewed (since a binocular is generally used for viewing a distance object, there is a high possibility that an obstacle passes as described above), if the automatic focusing mechanism works so that the binocular is focused on the obstacle, the binocular would be greatly inconvenient.
Further, in the binoculars according to the above-mentioned prior arts, a motor is employed as a driving source for driving a focusing lens, and a battery is mounted as the power source of the motor.
Since the motor cannot work at a supply voltage lower than a predetermined voltage, there is a possibility that the motor stops at an undesirable point in an automatic focusing operation. In this case, since the lens cannot be moved to a desirable position in a binocular which does not have a manual focusing function, there are frequent occasions when the binocular cannot work as binocular according to a position where the lens stops (especially, a near focusing position).
Further, in Japanese published Patent Application No. S62-6205 and Japanese published Patent Application No. S60-46407, a focusing speed is not recited.
If a focusing speed is too low, not only a chance to view an object is lost, but also the eye becomes fatigued since human eye's own focusing function works. That is, in trying to carefully view an object, human eye tries to obtain an in-focus condition by changing a curvature of its crystalline lens within a limited range (different according to visual acuity and ages). The adjusting ability of human eye largely depends on ages as shown in FIG. 38 (FIG. 38 corresponds to FIG. 104 on page 56 of "Science of the Eye" by Masao Nita, second edition of the revision, published by Bunkoudou Ltd.). Although it is more than 10 diopters at the age of approximately 10, gradually decreased by the aging, it becomes approximately 1 diopter at the age of 60 or thereabout. Therefore, human eye which has the adjusting power tries hard to adjust the focus, and human eye which does not have the adjusting power tries even harder to adjust the focus. In either case, it would be a great strain for the eye.
Therefore, in a binocular where although the automatic focusing function is provided, the focusing function by human eye works before the automatic focusing function works, stress (tension) builds up on the eye, and such a binocular is not preferable especially in viewing for a long time period, since strain of the eye accumulates.
Further, in Japanese published Patent Application No. S62-6205 and Japanese published Patent Application No. S60-46407, an in-focus range is not specifically described.
For example, since the continuous automatic focusing is appropriate for, for example, viewing a moving object with a binocular having the automatic focusing function, a binocular is usually provided with the continuous automatic focusing function. However, since a binocular has a long focal length, a binocular shake is easily caused in a continuous automatic focusing operation, so that a measured distance is apt to vary widely by the binocular shake. Moreover, there are occasions when a variation in measured distance is caused by a noise, etc. to a circuit. When there is a variation in measured distance, a so called hunting phenomenon is caused in a lens driving operation. Therefore, some binoculars have a relatively large in-focus range in order to reduce the variation in measured distance.
Moreover, the farther away an object is situated, the lower its contrast becomes, which causes errors in measuring a distance. Therefore, it is desirable that the in-focus range is varied according to an object distance and that the in-focus range is large when an object is situated far away.
When the in-focus range is large, however, since the lens is not moved when the in-focus condition is not actually obtained due to a movement of an object, the view through the binocular is more or less out of focus, or since the eye is forced to adjust the focus, the eye becomes fatigued. And, when the in-focus position is situated at an end of the in-focus range, the above problems are especially conspicuous. Moreover, for the elderly, although an automatic focusing operation is performed, the view through the binocular is inevitably out of focus.
Further, Japanese published Patent Application No. S62-6205 and Japanese published Patent Application No. S60-46407 do not disclose binoculars having the automatic focusing function and a manual power focusing function. On the other hand, in the art of a camera, it is generally known to provide an automatic focusing function and power focusing function. In some cameras, a selection of these functions is made by a mode changing switch.
Generally, the automatic focusing function is not effective for all objects since it does not work effectively on some objects. For example, objects having no contrast or reflecting would not be suitable for the automatic focusing. Therefore, in order to view such objects under an excellent focusing condition, it is desirable to provide a binocular with the power focusing function.
Although it is considered that the frequency of use of the power focusing function is fairly lower than that of the automatic focusing function, operation members for the power focusing function are arranged at a place convenient for operating them since an excellent operationability is required for them. However, if the operation members for the manual power focusing function are provided at a place convenient for operating them, a possibility increases that an undesirable interruption is applied during an automatic focusing operation by operating the members by mistake. On the other hand, to employ a changing switch like in the cameras of the above-mentioned prior arts is inconvenient in another sense (that is, in a sense that the number of operations increases).